Aftertreatment System Using LNT and SCR

ABSTRACT

A multiple catalyst system for the treatment of exhaust gases generated by a combustion engine in an after treatment system. The system includes a substrate and a plurality of catalysts. The plurality of catalysts may provide a plurality of catalyst sections. A variety of different catalysts, as well as a variety of different combinations of catalysts, may be employed. Catalysts may include both catalysts that assist in the removal of nitrogen oxides from the exhaust gas during cold start operation of the engine, as well as catalysts that are generally more effective in removing nitrogen oxides after the temperature of the after treatment system and/or exhaust gas is elevated to normal operating temperatures.

RELATED APPLICATIONS

This application claims priority to U.S. application Ser. No.61/737,199, having a filing date of Dec. 14, 2012, which is incorporatedherein by reference.

BACKGROUND

Combustion engines may employ emission controls or systems that areconfigured to reduce the amount of nitrogen oxides (NO_(x)), carbondioxide (CO₂), and/or iron (Fe), among others, which are present in theengine's exhaust gas. For example, one aspect of controlling suchemissions may include the use of a Selective Catalytic Reduction (SCR)system. The SCR typically uses a catalyst and a reductant to convert NOin the exhaust gas into at least nitrogen gas and water. The reductantmay be a liquid or gas, such as, for example, urea, anhydrous ammonia,or aqueous ammonia, among others.

In some systems, the ammonia reductant is molecularly bounded to a solidhost salt that is placed inside a metallic vessel, which may be referredto as a main unit. Through an exothermic reaction, the ammonia reductantis released from the host salt in a gaseous state. Initiation of such anexothermic reaction may be provided by the flow of hot engine coolant,which may circulate through a heating mantle surrounding the main unit.Moreover, in such systems, engine heat transferred to coolant throughconduction may provide heat that aids the exothermic reaction thatreleases the ammonia reductant from the solid host salt.

Such SCR systems may also be used with an ammonia oxidizing catalyst(AMO_(x)). The AMO_(x) system is typically configured to prevent ammonia(NH₃) that had been injected into the exhaust gas, but was not used bythe SCR system in the conversion of NO_(x), from slipping out of theafter treatment system. For example, the AMO_(x) system may include acatalyst, such as, for example, a zeolite-based and alumina-supportedmetal or metal oxide catalyst, that converts at least a portion of theammonia remaining in the exhaust gas into nitrogen gas (N₂) and water(H₂O). However, in at least some systems, the effectiveness of theAMO_(x) system may be impacted by the temperature of the exhaust gas,and more specifically, whether the exhaust gas has been heated totemperatures that promote the ability of the AMO_(x) system to convertthe ammonia into nitrogen gas and water.

For example, SCR systems are not typically effective in controllingNO_(x) emissions at temperatures below 200° Celsius, which may presentissues with compliance with emissions regulations. Additionally, thedosing capability of the SCR system may require exhaust gas temperaturesto be above 190° Celsius so as to prevent reductants injected into theexhaust gas from being deposited on the SCR system. Therefore, aftertreatment systems that rely on an SCR system alone for the removal ofNO_(x) generally require a relatively quick elevation of exhaust gastemperature in order to be effective.

To attain the elevated temperatures needed for the SCR system, enginesused with after treatment systems having only SCR systems for thetreatment of NO may operate in a warm up mode in which the engine isrunning in richer condition with more fuel consumption so as to generateheat necessary for the effective operation of the SCR system. However,higher fuel consumption is directly related to an increase in CO₂production, and may also increase the fuel consumption rate of engine.

Yet, with SCR systems, there may be a delay before the engine coolantreaches temperatures needed for the engine coolant to aid in theabove-discussed exothermic reaction. For example, a cold internalcombustion engine may require sufficient operation time following coldstart-up before the engine coolant is elevated to sufficienttemperatures (e.g. greater than 200° Celsius) to aid in this exothermicreaction. This delay in time may be further extended due to thetemperature of the surrounding environment, such as, for example, duringextreme cold weather conditions. Another type of catalyst based systemfor reducing the amount of NO present in the engine's exhaust gas is alean NO trap (LNT). The LNT is typically configured to remove NO fromthe exhaust gas by trapping or absorbing the NO_(x), such as, forexample, through the use of zeolite. The ability of the LNT to trap NOtypically is not dependent on the temperature of the exhaust gas beinggenerated by the operation of the associated engine. Thus, the LNT maybe at least partially effective during cold engine start-up conditionsand/or during operation of the engine in relatively cold environments.

BRIEF SUMMARY

According to certain embodiments, a multiple catalyst system is providedfor the treatment of exhaust gases by after treatment system. The systemincludes a common substrate and a plurality of catalysts. The pluralityof catalysts may provide a plurality of catalyst sections.

Additionally, according to certain embodiments, a multiple catalystsystem is provided for the treatment of exhaust gas in an aftertreatment system. The multiple catalyst system includes a first catalystapplied to a portion of a substrate, the first catalyst including a leanNO_(x) trap catalyst. Further, a second catalyst is applied to a portionof the substrate, the second catalyst including a selective catalyticreduction catalyst.

Embodiments also provide a multiple catalyst system for an exhaust gasafter treatment system that includes a common substrate having aplurality of catalyst sections. Each of the plurality of catalystsections includes at least one catalyst formulated to treat exhaust gasgenerated by a combustion engine.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates a multiple catalyst system that includes a singlecommon substrate having multiple catalysts.

FIG. 2 illustrates a multiple catalyst system that includes substrateportions that are joined together and which have multiple catalysts.

DETAILED DESCRIPTION

FIG. 1 illustrates a multiple catalyst system 10 that includes a singlecommon substrate 18 having multiple catalysts 12, 14, 16, and which arehoused in a housing 20. The housing 20 may be operably connected to anexhaust gas treatment system. Additionally, the housing 20 may have afirst end 28 that is operably connected to a pipe or line, such as by aclamp or press fit, among others, that allows exhaust gas to bedelivered to the interior 32 of the housing 20. The second end 30 mayalso be operably connected to the exhaust gas treatment system so thatexhaust gas may exit the interior 32 and continue flowing to othercomponents of the exhaust treatment system, such as through piping orlines that are operably connected to the second end 30.

At least a portion of the catalysts 12, 14, 16 used with the substrate18 may be different than other catalysts 12, 14, 16 used with thesubstrate 18. The catalysts 12, 14, 16 may provide, either alone or incombination, one or more catalyst sections 20, 22, 24 on the substrate18. For example, referencing FIG. 1, a first catalyst 12 applied to aportion of the substrate 18 provides a first catalyst section 24, whilea second catalyst 14 applied to at least another portion the substrate18 provides a second catalyst section 24, and a third catalyst 16applied to at least another portion of the substrate 18 provides a thirdcatalyst section 26. The catalysts may be a catalyst and/or a catalystwashcoat.

The catalysts 12, 14, 16, and their associated catalyst sections 22, 24,26, may be arranged on portions of the substrate 18 such that thecatalyst sections 22, 24, 26 abut against, overlap, and/or are spacedapart from the adjacent catalyst section(s) 22, 24, 26. For example, forat least purposes of illustration, the three catalyst sections 22, 24,26 shown in FIG. 1 are in an abutting arrangement on the substrate 18.However, application of the catalysts 12, 14, 16 to the substrate 18 mayresult in at least some degree of overlapping of at least some of thecatalysts 12, 14, 16, and thus overlapping of the associated catalystsections 22, 24, 26.

According to certain embodiments, the catalysts 12, 14, 16 may bearranged in a particular order on the substrate 18 so that exhaust gasfrom the operation of an internal combustion engine flows over thecatalysts 12, 14, 16 in a particular order. For example, thearrangement, order, and selection of catalysts 12, 14, 16, may besuitable for after treatment of the exhaust gases during different typesof engine operating conditions or periods of operation. Accordingly, avariety of different catalyst formulations and combinations of catalystmay be applied to the substrate 18. For example, the catalysts 12, 14,16 may be selected for after treatment of NO_(x) in the exhaust gas byat least some of the catalysts 12, 14, 16 during cold startingconditions, such as when engine exhaust gas temperatures are belowapproximately 200° Celsius, as well as treatment of the exhaust gases bythe same and/or different catalysts 12, 14, 16 on the substrate 18during normal operating conditions, such as when the temperature of theexhaust gas is above 250° Celsius. For example, the embodimentillustrated in FIG. 1 provides a first catalyst section 22 that that hasan LNT catalyst as the first catalyst 12, an SCR catalyst as the secondcatalyst 14 of the second catalyst section 24, and a third catalyst 16that is an AMO_(x) catalyst for the third catalyst section 26.

According to such embodiments, during cold starts, the temperature ofthe exhaust gas and/or temperature of the after treatment system istypically relatively low. Such low temperature conditions may cause SCRcatalysts to be generally less effective than when operating undernormal, elevated exhaust gas temperature conditions. However, under suchlow temperature operating conditions, an LNT catalyst may still beeffective in removing NO_(x) from the exhaust gas to meet a desiredmaximum level of NO_(x) reduction. Accordingly, the system 10 may bearranged such that the LNT catalyst is positioned on the substrate 18upstream of an SCR catalyst, such as, for example, the LNT catalystbeing the first catalyst 12 shown in FIG. 1 to be exposed to exhaust gasdelivered through the first end 28 of the housing, while the SCRcatalyst is the second catalyst 14. According to such an arrangement,the SCR catalyst may be able to remove at least a portion of the NO_(x)in the exhaust gas that was not removed by the LNT catalyst. Therefore,according to certain embodiments, the LNT catalyst and the SCR catalystare sized and proportioned to provide, together, sufficient NO_(x)reduction during cold start conditions. Further, the quantity of NO_(x)removed by the SCR catalyst may increase as the temperature of theexhaust gas and/or the temperature of the after treatment systemincreases.

According to such embodiments, as the temperature of the exhaust gasand/or after treatment system increases, NO_(x) that had been stored bythe LNT catalyst for the first catalyst 12 may begin to be released fromthe first catalyst 12. However, typically, when such temperatureincreases are attained, the SCR catalyst, such as the second catalyst 14in the illustrated embodiment, has warmed up sufficiently that the SCRcatalyst is able to provide the requisite NO_(x) reduction to reduceboth the NO_(x) being released from the LNT catalyst and the NO_(x) thatis present in the exhaust gas that is passing by the second catalyst 14.

As previously discussed, generally, SCR systems are generally noteffective in NO_(x) conversion until exhaust gas temperatures and/ortemperatures in the interior 32 of the housing are above 250° Celsius.However, the use of one or more substrates that contain both a catalystthat is effect at treating NO_(x) in exhaust gases at cold temperatures,such as an LNT catalyst, and a catalyst that is effective in convertingNO_(x) at normal operating temperatures, allows the system 10 toeffectively remove and/or convert NO_(x) in exhaust gases at both coldand normal operating temperatures. As an AMO_(x) catalyst may be used toremove ammonia (NH₃) that had been injected into the exhaust gas but wasnot used by the SCR catalyst, the AMO_(x) catalyst may be positioneddownstream of the SCR catalyst on the substrate 18, such as the thirdcatalyst 16 being the AMO_(x) catalyst. Additionally, the AMO_(x)catalyst may be configured to reduce the ammonia passing by the AMO_(x)catalyst to sufficiently low levels at all expected operatingtemperatures.

FIG. 2 illustrates an embodiment of a multiple catalyst system 50 thatincludes a substrate 52 having first and second substrate portions 54,56 that are housed within an interior 64 of a housing 58. Similar to thehousing 20 discussed above with respect to FIG. 1, the housing 58 shownin FIG. 2 may also include first and second ends 60, 62 that areoperably connected to an exhaust gas after treatment system.

According to certain embodiments, the first and second substrateportions 54, 56 may be part of a common, single substrate 52. Accordingto other embodiments, the first and second substrate portions 54, 56 maybe from separate substrates that are either physically adjoined togetherafter being coated with one or more catalysts or positioned in relativeclose proximity to each other, such as, for example, in an abutting,non-abutting, or overlapping orientation.

According to certain embodiments, the first substrate portion 54includes a first catalyst section 66 that contains one or morecatalysts, such as, for example, first and second catalysts 68, 70. Inthe illustrated embodiment, the first catalyst 68 is shown as being anLNT catalyst, while the second catalyst 70 is shown as being an SCRcatalyst. However, the first catalyst section 66 may include a varietyof different catalysts and/or combinations of catalysts. Similar to thecatalysts 14, 16, 18 discussed above with respect to FIG. 1, the firstand second catalysts 68, 70 shown in FIG. 2 may also be positioned inthe first substrate portion 54 in overlapping or abutting orientations,or may be separated from each other on the first substrate portion 54.

Similar to the first substrate portion 54, the second substrate portion56 may also include a second catalyst section that includes one or morecatalysts. For example, the embodiment shown in the FIG. 2 illustratesthe second catalyst section 72 as having third and fourth catalysts 74,76.

The use of separate substrate portions 54, 56 the second and thirdcatalysts 54, 56 may facilitate the coating of the substrate 52 with oneor more of the catalysts, such as facilitating the coating of the mainSCR catalysts, illustrated in FIG. 2 as the second and third catalysts70, 74.

After being coated with their respective catalysts, the first and secondsubstrate portions 54, 56 may be joined together, such as, for example,by physically joining the first and second substrate portions 54, 56, orpositioning the first and second substrate portions 54, 56 in relativeclose proximity to each other.

1. A multiple catalyst system for the treatment of exhaust gas in anafter treatment system, the multiple catalyst system comprising: acommon substrate; and a plurality of catalysts applied to at least aportion of the common substrate to provide a plurality of catalystsections.
 2. The multiple catalyst system of claim 1, wherein themultiple catalyst system includes a plurality of common substratesjoined together, each of the plurality of common substrates having aplurality of catalysts.
 3. The multiple catalyst system of claim 1,wherein the plurality of catalysts include a first catalyst, a secondcatalyst, and a third catalyst.
 4. The multiple catalyst system of claim3, wherein the first catalyst is a lean nitrogen oxide trap catalyst,the second catalyst is a selective catalytic reduction catalyst, and thethird catalyst is an ammonia oxidizing catalyst.
 5. The multiplecatalyst system of claim 4, wherein the lean nitrogen oxide trapcatalyst is positioned upstream of the selective catalytic reductioncatalyst on the substrate, and the selective catalytic reductioncatalyst is positioned upstream of the ammonia oxidizing catalyst on thesubstrate so that exhaust gas in the after treatment system firstencounters the lean nitrogen oxide trap catalyst before encountering theselective catalytic reduction catalyst.
 6. The multiple catalyst systemof claim 4, wherein at least at least two of the plurality of catalystsections abut each other.
 7. The multiple catalyst system of claim 1,wherein the substrate includes a first substrate portion and a secondsubstrate portion, a portion of the plurality of catalysts being appliedto at least a portion of the first substrate portion, and the remainingplurality of catalysts applied to at least a portion of the secondsubstrate portion.
 8. A multiple catalyst system for the treatment ofexhaust gas in an after treatment system, the multiple catalyst systemcomprising: a substrate; a first catalyst applied to a portion of thesubstrate, the first catalyst having a lean nitrogen oxide trapcatalyst; and a second catalyst applied to a portion of the substrate,the second catalyst having a selective catalytic reduction catalyst. 9.The multiple catalyst system of claim 8, wherein the first catalyst ispositioned upstream of the second catalyst so that exhaust gas passesthe first catalyst before reaching the second catalyst.
 10. The multiplecatalyst system of claim 9, further including a third catalyst appliedto a portion of the substrate and a fourth catalyst applied to anotherportion of the substrate, the third catalyst being a selective catalyticreduction catalyst, the fourth catalyst being an ammonia oxidizingcatalyst.
 11. The multiple catalyst system of claim 10, wherein thesubstrate includes a first substrate portion and a second substrateportion, wherein the first and second catalysts are applied at least aportion of the first substrate portion, and the third and fourthcatalysts are applied to at least a portion of the second substrateportion.
 12. A multiple catalyst system for an exhaust gas aftertreatment system comprising: a common substrate having a plurality ofcatalyst sections, each of the plurality of catalyst sections having atleast one catalyst formulated to treat exhaust gas generated by acombustion engine.
 13. The multiple catalyst system of claim 14, whereinthe plurality of catalysts sections include a first catalyst sectionhaving a first catalyst and a second catalyst section having a secondcatalyst, the first and second catalysts formulated for the removal ofnitrogen oxides from exhaust gas, wherein the first catalyst is moreeffective than the second catalyst in removing nitrogen oxides fromexhaust gas generated during cold start engine conditions, and whereinthe second catalyst is more effective than the first catalyst inremoving nitrogen oxides from exhaust gas during normal engine operatingtemperatures.
 14. The multiple catalyst system of claim 13, furtherincluding a third catalyst, and wherein the first catalyst is a leannitrogen oxide trap catalyst, the second catalyst is a selectivecatalytic reduction catalyst, and the third catalyst is an ammoniaoxidizing catalyst.
 15. The multiple catalyst system of claim 14,wherein the lean nitrogen oxide trap catalyst is positioned upstream ofthe selective catalytic reduction catalyst on the substrate, and theselective catalytic reduction catalyst is positioned upstream of theammonia oxidizing catalyst on the substrate so that exhaust gas in theafter treatment system first encounters the lean nitrogen oxide trapcatalyst before encountering the selective catalytic reduction catalyst.